Electroacoustic transducer calibration method and apparatus

ABSTRACT

Methods and apparatus for calibrating terminated ultrasonic electroacoustic transducers are disclosed. The transducers are excited by a driving signal to emit an ultrasonic acoustic wave, which wave is received either by the same transducer after reflection or by a second transducer to produce an output signal. The driving signal and output signal are supplied to measuring circuitry deriving transd 
     BACKGROUND OF THE INVENTION 
     The invention described herein was made in the course of work under a grant or award from the Department of Health, Education and Welfare.

BACKGROUND OF THE INVENTION

The invention described herein was made in the course of work under agrant or award from the Department of Health, Education and Welfare.

The present invention relates, in general, to calibration techniques forelectroacoustic transducers, and more particularly to apparatus andmethods for calibrating ultrasonic transducers of the type useful inbiomedical applications.

A precise knowledge of the performance characteristics such asbandwidth, sensitivity, resonance frequency, the field intensitypattern, and the like, of a transducer device when it is operated in itstransmit and receive modes is a prerequisite to effective use of thedevice and to the optimization of transducer design. Accordingly, thecharacterization of ultrasonic transducers through the use of areciprocity calibration technique is attractive, since it requires nosecondary ultrasonic standards against which acoustic measurements arecompared. Only electrical measurements are involved, which directlyrefer back to primary electrical standards (see W. R. MacLean, "AbsoluteMeasurement of Sound Without a Primary Standard", J. Acoust. Soc. Amer.,Vol. 12, pp 140-146, 1940.) A self-reciprocity calibrating techniquedescribed by E. L. Carstensen, "Self-reciprocity Calibration ofElectroacoustic Transducers", J. Acoust. Soc. Amer., Vol. 19, pp.961-965, 1947, employs a single transducer, alternately used as aburst-excited transmitter and receiver, along with a 100 percentreflector. By this technique one can directly observe the microphone andspeaker sensitivity of a transducer, as the ratio of open circuit outputvoltage and driving current, or the ratio of short circuit outputcurrent and driving voltage. However, a transducer model is needed toextrapolate from these measurements the response of the terminatedtransducer.

While this technique has served the sonar field as an excellent standardof calibration, as established by the ANSI Standard S1.20-1972, entitled"Procedures for Calibration of Underwater Electrocoustic Transducers",and as set forth in Underwater Acoustics Handbook-II, V. M. Albers,University Park, Pa.: Pennsylvania State University Press, 1965, Chapter21, as well as in Underwater Electroacoustic Measurements, R. J. Bobber,Washington, D.C.: Naval Research Laboratory, 1970, Chapter 2, thetechnique has not found acceptance by authors in the ultrasonic field.

Various factors have prevented reciprocity calibration techniques frombeing generally adapted as measurement standards for ultrasonictransducers. For example, open circuit and short circuit measurements,which can be made at relatively low frequencies, are not easily attainedin the high frequency domain. Stray effects and loading introducesignificant errors, and separate loss measurements would be required tocompensate for these errors. (see R. J. Bobber, ibid.) Furthermore,since ultrasonic transducers are always operated with an electricaltermination, an ideal voltage or current source description is only ofuse if the equivalent output impedance is known. This impedance is afunction of many variables (transducer material, geometry, mode ofresonance, mounting and loading) and no simple model is available torelate these variables to an equivalent impedance.

Electronic impedance matching, of only incidental interest at lowerfrequencies, is an important engineering tool in the megahertz range. Onthe transmitter side it allows a transducer current drive to be realizedwith a minimal source voltage. On the receiver side it increases thetransducer's signal output and optimizes the signal to (amplifier) noiseratio. Factors like bandwidth, sensitivity, and frequency of resonanceare affected by the termination, and a designer will generally select aspecific termination to meet specific design criteria. It is thus highlydesirable to measure the performance of an ultrasonic transducer inabsolute terms under the conditions of actual termination. Ideally, sucha technique should also be simple enough to be incorporated inbiomedical ultrasonic equipment, to allow users to verify properoperation of apparatus under conditions of actual use. This applicationdiscloses novel options for such calibration standards.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a novelmethod and apparatus for calibrating electroacoustic transducers solelyby electronic measurements, without relying upon derived acousticalstandards.

It is a further object of the present invention to provide a method andapparatus for calibrating ultrasonic electroacoustic transducers underrealistic end-use conditions, with an applied electrical termination andradiating into a medium similar to the end use conditions.

It is another object of the present invention to provide a method andapparatus for measuring the performance characteristics of anelectroacoustic transducer in absolute terms when operated at ultrasonicfrequencies and electrically terminated.

Another object of the present invention is the provision of a method andapparatus for measuring performance characteristics such as microphoneand speaker sensitivity, transducer gain, transducer sensitivity,acoustic pressure and plane wave acoustic intensity, under actual enduse conditions wherein the transducer is electrically terminated.

Briefly, the present invention is directed to a method for determining,under a variety of test conditions, the performance characteristics of atransducer. In accordance with the method, measurements are made of theelectrical input voltage to the transducer in its transmit mode and theelectrical output voltage or current from the transducer in its receivemode, and these measurements are used to derive quantities such asterminated microphone and speaker sensitivity, transducer gain and itsderivative transducer sensitivity, and plane wave field intensity. Themethod steps are carried out, in the preferred form of the invention, bymeans of a test apparatus which includes an ultrasonic waterbath testfixture in which the transducer is mounted. The test fixture contains areflector which is mounted for movement about the transducer, so that itcan be selectively located with respect to the axes of symmetry of thetransducer field pattern, which is the pattern of transducerdirectivity. The reflector is positioned to reflect the segment of thetransducer field under investigation back onto the transducer.

The transducer is connected in a transmit mode to electrical circuitrywhich includes a driving source that produces a drive signal in the formof a burst having a duration long enough to assure steady-stateexcitation of the transducer, but shorter than the acousticaltransmit-receive delay, which is the time required for the transmittedsignal to travel to the reflector and back to the transducer. At the endof the excitation burst, the electrical circuitry switches to a receivemode to detect the returned acoustic signals. The electrical circuitryprovides, in the receive mode, a terminating impedance for thetransducer which is the same as the driving source impedance during thetransmit mode. This may be accomplished, in the preferred form of theinvention, by means of a single impedance, but it could equally well beprovided with two identical impedances.

The receive circuitry includes a receiver amplifier which monitorseither the voltage or the current generated by the transducer uponreception of the transmitted and reflected burst of ultrasound. If thevoltage is to be monitored, it is measured across the resistive part ofthe terminating impedance; if the received current is to be monitored,then the voltage developed across a small current sensing resistor inseries with the transducer output is measured. A signal amplitudemeasuring circuit connected to the output of the receiving circuitryresponds, during the period where the received signal has reached a(quasi) steady state value, to produce a narrow-band average value ofthe A.C. peak-to-peak voltage of the monitored signal, which value willbe proportional to either the received voltage or the received current.This value is stored in a sample-and-hold memory element for display ona meter, chart recorder, or the like.

Upon completion of the measurement and sampling steps, the circuitryreturns to the transmit mode of operation in preparation for the nextburst-echo sequence, which can be executed as soon as the echos of thepreviously transmitted burst have died out in the waterbath. The nextsubsequent measurement is then used to update the value stored in thesample-and-hold memory element, and to update the display. A largenumber of measurements can be made in this manner with the measuredvalues being displayed or recorded as functions of a swept transducerfrequency, a varying acoustic field angle, reflector distance, or othervariables.

Additional switching means are included in the preferred embodiment ofthe invention to produce and display a measure of the open-circuitvoltage of the driving source, by connecting this source directly to thereceiving amplifier circuitry. This displays the signal amplitude of thedriving source, and thus provides a full scale value for the transducerparameter, such as sensitivity, intensity, or the like, which is beingmeasured. Particularly in those instances where the parameter ofinterest is obtained by dividing an output voltage or current by theopen-circuit driving voltage amplitude it is important to have bothamplitude measurements made by the same circuitry to eliminate gainvariations in the measuring circuits as a source of error.

In another form of the invention, separate ultrasonic transducers areused for the transmitter and the receiver, the two transducers beingaligned in a wave conducting medium for acoustic coupling. In thisarrangement, the transmitter can be continuously energized and thereceived signal continuously monitored. If the two transducers arereciprocal and identical, and the electrical terminations are madeidentical, then the same measurements of terminated microphone andspeaker sensitivity, transducer gain, transducer sensitivity, andacoustic intensity can be performed as in the above-described burst-echoarrangement, by measuring ideal source input voltage and output voltageor current. If the two transducers are nonreciprocal and/or different,the same quantities can be derived from electronic input and outputmeasurements by substituting different transducers in the samemeasurement configuration and eliminating transducer variables from theresults.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional objects, features and advantages of thepresent invention will become apparent to those of skill in the art froma consideration of the following detailed explanation of a preferredembodiment of the invention, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic illustration of terminated electroacoustictransducer system;

FIG. 2 is a diagrammatic illustration of a self-reciprocity calibrationsystem for a terminated transducer, according to the present invention;

FIG. 3 is a modification of the calibration system of FIG. 2, showingthe two-transducer electroacoustic transmit-receive chain of the presentinvention;

FIG. 4 is a block diagram of a circuit for measuring transducer gain andsensitivity in accordance with the present invention;

FIG. 5 is a block diagram of a circuit for measuring terminatedmicrophone and speaker sensitivity, acoustic pressure and intensity inaccordance with the present invention;

FIG. 6 is a block diagram of a circuit for calibrating electroacoustictransducers in the two transducer chain arrangement of FIG. 2; and

FIG. 7 is a block diagram of an A.C. voltage amplitude measuring circuitfor the calibration circuits of FIGS. 4, 5 and 6.

DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to a more detailed consideration of the present invention,attention will first be given to the theoretical basis for theultrasonic transducer calibration method to be described. With only afew exceptions, as outlined by E. M. McMillan in "Violation of theReciprocity Theorem in Linear Passive Electromechanical Systems", J.Acoust. Soc. Amer., Vol. 18, pp. 344-347, 1946, electroacoustictransducers capable of operating as both transmitter and receiverconform to a reciprocity theorem, which establishes a relation betweentheir performance as transmitter and receiver. The reciprocity relationsof terminated transducers may be derived from MacLean's originalformulation of this theorem (ibid), proven for the general case by L. L.Foldy and H. Primakoff in their article entitled "A General Theory ofPassive Linear Electroacoustic Transducers and the ElectroacousticReciprocity Theorum. I", J. Acoust. Soc. Amer., Vol. 17, pp. 109-120,1945. It follows, therefore, that whenever the conventional reciprocitystatements hold, the extensions are valid too.

As illustrated in FIG. 1, an arbitrary electrical network driving areciprocal transducer 10 when the transducer is transmitting (i.e. isacting as a speaker) can be replaced by the Thevenin-equivalent idealvoltage source 12, having a voltage E_(in), and a series impedance 14,having an impedance Z_(t). The transducer may be any suitable devicecapable of responding to a high-frequency drive signal to produce anacoustic output wave at ultrasonic frequencies, and may be, for example,a conventional piezoelectric device, available from Transducer Products,Goshen, Conn. A switch 16 shifts the transducer from the transmit modein which it is connected to the source 12 by way of transmit terminal18, to a receive mode, which is accomplished by shifting switch arm ofswitch 16 to the receive terminal 20. This replaces the source 12 with ashort circuit line 22, leaving the impedance 14 in the circuit, so thatthe terminating impedance is the same in both the transmit and receivemodes. This terminating impedance 14 may be considered a part of thetransducer, in which case the microphone sensitivity M and the speakersensitivity S of the transducer may be defined as follows:

    M(Z.sub.t)=(I.sub.out /p.sub.ff).sub.E.sbsb.in.sub.=0      (1)

    S(Z.sub.t)=(p.sub.fc /E.sub.in).sub.p.sbsb.b.sub.=0        (2)

where p_(ff) is the free field acoustic pressure that would exist at thelocation of the face of a receiver if the receiver were not there,p_(fc) is the free-field pressure at a calibration distance from atransmitter, I_(out) is the current flow generated at the output of thereceiving transducer, and p_(b) is the pressure at the transducer facewith all surface motion blocked. These definitions are identical toMacLean's short-circuit sensitivities, although in the above expressionsthe short-circuit suffix has been omitted since the transducer 10 isactually terminated differently than MacLean's.

M and S in the foregoing expressions (1) and (2) include the effect ofthe termination: for every different termination, a different value isfound for the sensitivities. The two partial conditions imply that thetransducer is operated only as transmitter in the definition of S, onlyas receiver in the definition of M. MacLean's reciprocity relation nowstates that the ratio of microphone and speaker sensitivity isindependent of the transducer (thus independent of the value of theterminating impedance, as long as the same termination is used for thetransmit and the receive mode of operation):

    M(Z.sub.t)/S(Z.sub.t)=J                                    (3)

The reciprocity constant J, which is the transfer admittance of themedium, describes the acoustical coupling between transmitter andreceiver (See R. J. Bobber, "General Reciprocity Parameter", J.Acoustical Soc. Amer., Vol. 39, pp. 680-687, 1966). Far-fieldexpressions for various types of waves are

    J.sub.s =2d.sub.cal λ/Z.sub.o (spherical waves)     (4a)

    J.sub.c =2L(d.sub.cal λ).sup.1/2 /Z.sub.o (cylindrical waves) (4b)

    J.sub.p =2A/Z.sub.o (plane waves)                          (4c)

where d_(cal) is a standardized calibration distance, L is the length ofa line transducer, A is the effective transducer radiating area, Z_(o)is the plane wave acoustical impedance, and λ is the acousticwavelength.

Implicit in expression (3) is the notion that M and S are only directlyrelated to each other by J if the terminating impedance is the same forboth. MacLean (ibid) chooses this impedance equal to zero. Hisstatements hold, however, for arbitrary termination.

(A) Self-reciprocity Calibration Methods

Based on the foregoing, equations expressive of the characteristics of atransducer operated in a self-reciprocity mode can be derived.Accordingly, the transducer 10 is operated in a burst-echo pattern,where the transducer is excited by the drive source 12 to transmit ashort burst of acoustic waves, generally indicated at 24 in FIG. 2,after which the transducer is shifted to receive the returning acousticsignals echoed from a reflector surface 26. The duration of the soundbursts is chosen to be long enough to assure a steady-state sinewaveexcitation of the transducer, and short enough to prevent overlap withthe returning echo, so that the transmit and receive operations areseparated in time. In addition, the electrical termination 14 is chosento be the same in both the transmit and receive modes.

Part of the emitted acoustic field 24 is reflected back onto thetransducer surface by the 100% reflecting surface 26. The reflector ispositioned at a distance d/2 from the transducer face, and can be movedabout the transducer for field pattern measurements. In the arrangementof FIG. 2, d is the distance to the mirror image 28 of the transducer,while in other calibrating configurations (to be described) d may be thedistance between two transducers. The free-field pressure at thelocation of the transducer (receiver) is therefore equal to the emittedpressure at a distance d. The appropriate expression for J establishesthe ratio of these pressures p_(ff) at d, and p_(fc) at d_(cal) :

    P.sub.fc /p.sub.ff =(d/d.sub.cal).sup.m,                   (5)

where the exponent m, which reflects the far-field symmetry of a wave,equals 1 for spherical, 1/2 for cylindrical, and 0 for plane waves.

Five equations are thus available, with five variables: M, S, p_(ff),p_(fc) and J, and various relationships can be derived from theseequations which will be expressive of various transducercharacteristics, as follows:

(i) Microphone and Speaker Sensitivity Self-reciprocity Calibration

The microphone (transmitter) and speaker (receiver) sensitivitycharacteristics of transducer 10 can be obtained in the self-reciprocitycalibration mode by solving expressions (1)-(5) for M and for S. Thisyields:

    M.sup.2 (Z.sub.t)=(I.sub.out /E.sub.in) (d/d.sub.cal).sup.m J (6)

    S.sup.2 (Z.sub.t)=(I.sub.out /E.sub.in) (d/d.sub.cal).sup.m /J (7)

These expressions permit a determination of the terminated microphoneand speaker sensitivities from the measurement of two electricalparameters, input voltage and output current, from the measurement of adistance, and from the determination of a constant (J) which is specificto the medium in which the transducer is operated. Although expressions(6) and (7) are similar to those developed by E. L. Carstensen (ibid),they differ in that Carstensen's equations pertain to an open-circuitedtransducer only, whereas (6) and (7) hold for any termination, and canbe used, for example, to establish a 50 ohm terminated calibrationstandard. The d/d_(cal) multiplier in (6) and (7) corrects a measurementwhich is made at some acoustical pathlength d to a standardized distanced_(cal).

(ii) Transducer Gain and Sensitivity Self-Reciprocity Calibration

Transducer gain, which is the inverse of transducer loss, is defined asthe ratio of power actually received in a load to the maximum power thatcan be derived from a source. (see "IRE Standards on Transducers:Definitions of Terms, 1951", Proceedings of the IRE, Vol. 39, pp.897-899, 1951). For a two-transducer electroacoustic transmit-receivechain of the type illustrated in FIG. 3, the gain G may be expressed as:

    G=4(I.sub.out.sup.2 /E.sub.in.sup.2) R.sub.S R.sub.L       (8)

where R is the real part of a terminating impedance, R_(S) =Re(Z_(s)),and R_(L) =Re(Z_(L)), Z_(S), Z_(L), and Z_(t) being equivalentelectrical terminating impedances. If equations (1), (2) and (5) arecombined with (8), then:

    G=4M.sub.1.sup.2 (Z.sub.S)S.sub.2.sup.2 (Z.sub.L) (d.sub.cal /d).sup.2m R.sub.S R.sub.L                                           (9)

where subscripts 1 and 2 identify transmit and receive functions,respectively.

The transducer chain of FIG. 3 includes the driving source 12 connectedthrough a terminating impedance 30, having impedance Z_(s), to atransmitting transducer 32. The acoustic waves produced by transmitter32 are coupled to a receiver transducer 34 by way of an acousticallyconductive medium 36, which may be a waterbath fixture. The electricaloutput produced by receiver transducer 34 is connected to a terminatingload 38 having an impedance Z_(L).

The acoustic transfer admittance J of the coupling medium 36 is the samefor both of the transducers 32 and 34, by virtue of the acousticreciprocity of the medium itself. Accordingly, with expression (3)equation (9) becomes:

    G=4(M.sub.1 S.sub.1 R.sub.S) (M.sub.2 S.sub.2 R.sub.L) (d.sub.cal /d).sup.2m                                                (10)

The transmit and receive functions appear in this expression fullyseparated:

    G=4T.sub.1 (Z.sub.S)T.sub.2 (Z.sub.L) (d.sub.cal /d).sup.2m (11a)

    T(Z.sub.t)=M(Z.sub.t)S(Z.sub.t)R.sub.t                     (11b)

where T is defined as the transducer sensitivity. T thus describes thepower transfer which occurs in a single electroacoustic conversion,including the effects of the acoustical dispersion of the medium. Theseparation of the transmit and the receive functions in theseexpressions permits individual calibration of the transducers 32 and 34.

Converting the foregoing equations for the gain of a transducer chain toobtain the gain in the self-reciprocity arrangement of FIG. 2, bycombining equations (8) and (11) yields:

    G=4I.sub.out.sup.2 R.sub.t.sup.2 /E.sub.in.sup.2 =4T.sup.2 (Z.sub.t) (d.sub.cal /d).sup.2m                                     (12)

or

    T(Z.sub.t)=(I.sub.out R.sub.t /E.sub.in) (d/d.sub.cal).sup.m (13)

In summary, the transducer sensitivity as defined in equation (11b) fora transducer chain can be measured in a self-reciprocity calibrationapparatus, by measuring the voltage developed across the terminatingresistance, and dividing that value by the output voltage of the idealdriving source 12. Further, since the power transfer in anelectroacoustic transmit-receive system follows directly from thetransducer sensitivities of its components, as set forth in equation(11a), then equations (11a) and (13) can form a convenient basis forcalibrating ultrasonic transducers.

When the voltage of the driving source 12 is kept constant duringmeasurements, a transducer can be characterized in absolute terms simplyby observing an output voltage, while varying the source resistance,varying the frequency of the driving voltage, or varying the angle ofreflection θ from reflector 26. Again, the ratio d/d_(cal) in equation(13) serves to correct a measurement made at an acoustical pathlength dto a standardized distance d_(cal), so that this ratio in the gainequations converts the microphone, speaker and transducer sensitivities,defined at d_(cal), to the actual measuring distance d. Often, d can bechosen to be equal to d_(cal), and the distance ratio can then beomitted from the calculations.

If measurements made at one acoustical pathlength are to be extrapolatedto different distances, knowledge of the dispersion of the acousticalwave is an obvious necessity. If unknown, the dispersion can beestablished by measuring the transducer gain as a function of distance.

An advantage of the present invention is that no acoustical parametersneed be known in order to measure transducer sensitivity. However,compatibility of the reciprocity constants J of two transducers isrequired to permit prediction of their joint transducer gain from theirself-calibrated sensitivites.

(iii) Acoustic Pressure and Intensity Self-Reciprocity Calibration

The acoustic intensity of the wave emitted by a transducer can bemeasured in the self-reciprocity calibration arrangement of FIG. 2.Thus, the acoustic field pressure p_(ff) at a distance d from thetransducer 10 can be obtained from equations (1)-(5):

    p.sub.ff.sup.2 =(I.sub.out E .sub.in /J) (d.sub.cal /d).sup.m (14)

A transducer's far-field intensity F at a distance d equals p_(ff) ²Z_(o). Expressions for the intensity are found by combining thisrelationship with equations (4a)-(4c):

    F.sub.s =I.sub.out E.sub.in /2dλ                    (15)

    F.sub.c =I.sub.out E.sub.in /2L(dλ).sup.1/2         (16)

    F.sub.p =I.sub.out E.sub.in /2A                            (17)

for spherical, cylindrical and plane waves, respectively. One can thuselectrically determine a transducer's emitted far-field intensity byexecuting a self-reciprocity measurement as in FIG. 2. This intensity isobserved as the current delivered in the receive mode of operation,times the driving open source voltage, times a geometry-relatedconstant. Intensity radiation patterns are measured by changing theangle of reflection, and intensity bandwith by varying the frequency.The voltage-current product in these equations differs from the ordinaryelectrical power expression. Voltage and current do not coincide intime, and their phase relation expresses the acoustical delay of themedium instead of a real/reactive power ratio.

(B) Other Calibration Methods

Other state-of-the-art calibration techniques can be combined with thetheory disclosed above, without departing from the scope of thisinvention. For instance, if two identical reciprocal transducers,terminated with the same impedance, are placed in each other's field,all measurement techniques derived under section (a) above can beapplied, but now often in continuous wave mode, rather than burst-echomode.

Another example is a conventional three-transducer calibration scheme,which can be modified to measure transducer sensitivity as disclosed inthe theory disclosed above. In this scheme, measurements of signaltransfer are made employing three different pairs of transducers out ofa set of three, by placing each pair in the same acoustic test circuitat the same transducer locations. Accordingly:

Measurement a: Transducer 1 transmits, Transducer 2 receives;

Measurement b: Transducer 1 transmits, Transducer 3 receives;

Measurement c: Transducer 2 transmits, Transducer 3 receives;

and employing for the three transducer terminating impedances, havingindices 1, 2 and 3, respectively, which impedances could be chosen equalto each other, the three transducer gain expressions are, per Eqs. (11a)and (12):

    G.sub.a =4T.sub.1 (Z.sub.1)·T.sub.2 (Z.sub.2)·(d.sub.cal /d).sup.2m =4(I.sub.out,a R.sub.2 /E.sub.in,a).sup.2      (18)

    G.sub.b =4T.sub.1 (Z.sub.1)·T.sub.3 (Z.sub.3)·(d.sub.cal /d).sup.2m =4(I.sub.out,b R.sub.3 /E.sub.in,b).sup.2      (19)

    G.sub.c =4T.sub.2 (Z.sub.2)·T.sub.3 (Z.sub.3)·(d.sub.cal /d).sup.2m =4(I.sub.out,c R.sub.3 /E.sub.in,c).sup.2      (20)

All I_(out) R_(i) /E_(in) ratios being established by measurements,three equations with three unknowns T₁ (Z₁), T₂ (Z₂) and T₃ (Z₃), areprovided. By elimination, the transducer sensitivity for all threetransducers follows. For instance, for T₁ :

    T.sub.1 (Z.sub.1)=(

    I.sub.out, a R.sub.02 /E.sub.in, a) (I.sub.out,b R.sub.3 /E.sub.in,b)(E.sub.in,c /I.sub.out,c R.sub.3) (d/d.sub.cal).sup.m (21)

In this calibration procedure only transducer 2 is used as bothtransmitter and receiver. Thus, it is possible to calibrate by thismethod non-reciprocal transducers, or investigate presumed reciprocity.

Similarly, acoustic intensity can be measured in such a three-transducermeasurement configuration. With three reciprocal transducers, there aresix combinations of transmitter and receiver. Keeping, for simplicity'ssake, the ideal driving source voltage the same in all cases, Eqs. (1)and (2) yield six equations of the form:

    (P.sub.fc,i).sup.2 =[I.sub.out,ij E.sub.in S.sub.i (Z.sub.i)]/M.sub.j (Z.sub.j')                                                (22)

where i,j=1, 2 or 3, but i≠j. The unknowns in these equations are thethree pressures emitted by the three transducers, their three terminatedspeaker sensitivites and their three terminated microphonesensitivities. Eq. (3) yields three additional relations between thesemicrophone and speaker sensitivities. From this total of nine equationswith nine unknowns, the free-field pressures and so the intensities canbe calculated. A further simplification in this procedure is possible bymaking the receiving terminating impedance equal to the transmittingsource impedance (Z=Z'). Then by reciprocity, the current received bytransducer i with transducer j transmitting equals the current receivedin transducer j with transducer i transmitting, so only threemeasurements are needed to perform this acoustic field calibration.

(C) Transducer Calibration Apparatus

The circuit arrangements for implementing the various calibrationmethods described above are very similar. In all cases, the open-circuitvoltage amplitude of the source driving the transducer is measured toobtain a scale calibration for the display device. In addition, eitherthe voltage developed across the resistive part of the receivingtransducer termination impedance is measured (to obtain the transducersensitivity), or the current flow through this termination impedance ismeasured (for the terminated microphone and speaker sensitivity, and theintensity determination). In the preferred form of the calibrationapparatus, the latter current measured is obtained with the aid of asmall sensing resistor through which the current flows so that thecurrent may be determined as a voltage measurement.

Turning now to FIG. 4, there is illustrated in block diagram form acircuit suitable for self-reciprocity transducer sensitivity calibrationin accordance with the present invention. This circuit includes a source50 of ultrasonic frequency voltage, which source may be a high frequencyoscillator or the like. One side of the oscillator 50 is connected to aground point 52, and the other side is connected by way of line 54 tothe transmit terminal 56 of a first selector switch 58 and to the scalecalibration terminal 60 of a second selector switch 62. Switches 58 and62 preferrably are solid state devices, but for convenience areillustrated by their mechanical equivalents.

Switch 58 is a transmit-receive selector which includes a movable switcharm 64 activated by suitable gating logic 66, connected to switch 58 byway of line 68, to shift the arm between the transmit terminal 56 and areceive terminal 70 which is grounded through line 71, and thus to shiftthe circuit and the transducer to be tested between a transmit mode anda receive mode. Similarly, the gating logic 66 provides a signal throughline 72 to a movable switch arm 74 of switch 62 to shift that armbetween the scale calibration terminal 60 and a measuring terminal 76,to thereby permit scale calibration for any display meters or recordingdevices used with the present system, and to permit measurement of thereceived transducer signals of interest, as will be further described.

The switch arm 64 of transmit receive selector switch 58 is connectedthrough a terminating impedance 80 having an impedance value R_(t),through line 82 to terminal 76, and through lines 82 and 84 to thedriving input of a transducer 86 under test. The ground terminal of thetransducer is connected by way of line 88 to a ground line 90 for thecircuits, which in turn is connected to ground point 52.

The switch arm 74 of switch 62 is connected by way of line 92 to connecta test signal representing the output of the transducer to the input ofan A.C. voltage amplitude measuring circuit 94. The measuring circuitalso receives a gating input from gating logic 66 by way of line 96, andreceives two phase-shifted standard signals from voltage source 50 byway of line 97. The measuring circuit is also connected by way of line98 to the ground line 90. An output line 100 from the measuring circuit94 carries a transducer performance calibration signal which may beconnected to a suitable display such as a chart recorder, meter or thelike, to produce a record of the measured transducer sensitivity.

In the circuit of FIG. 4, selector switch 62 is normally in the"measure" mode, wherein switch arm 74 is connected to the measuringterminal 76, as illustrated, thereby connecting the input line 92 of theamplitude measuring circuit 94 to the transducer 86 by way of line 84.To excite the transducer into its transmit mode, switch 58 is shifted bythe gating logic to cause switch arm 64 to be connected to the transmitterminal 56, whereby the drive signal from high frequency oscillator 50is connected through impedance 80 and lines 82 and 84 to drivetransducer 86. This causes the transducer to emit acoustic signals 102which are propagated through an acoustic medium within an acoustic testbath 104 toward a reflector 106. The waves are reflected at 106,producing an echo wave 108 which returns to the transducer 86.

The transducer 86 is excited into its transmit mode for a duration longenough to assure a steady state transducer excitation, but shorter thanthe acoustic transmit-receive delay; i.e. shorter than the time requiredfor the leading edge of the acoustic wave 102 to travel to the reflector106 and return to the transducer 86 in the form of echo 108. The gatinglogic controls the length of the transmit burst and terminates the burstby switching arm 64 from terminal 56 to the receive terminal 70. Thisdisconnects the high frequency oscillator and connects one end oftermination impedance 80 to the ground line 90 by way of receiveterminal 70 and lead 71, whereby current generated by transducer 86 uponreception of echo signal 108 will flow by way of lines 84, 82, impedance80 and line 71, to ground. This produces a test signal voltage E_(out)across impedance 80, which voltage is the transducer output voltage. Ata time selected by the gating logic 66, which time is adjustable througha transmit-receive distance adjustment in the gating logic, the A.C.voltage amplitude measuring circuit 94 samples and stores the steadystate value of the test signal voltage being generated by the transducerin response to the received echo signal 108, and appearing on line 92.This received voltage is equal to the resistance of impedance 80multiplied by the current flow therethrough, as expressed in Eq. (13).This sampled and stored value is made available on the output terminal100 for display, and is proportional to the transducer sensitivity, asshown in the theoretical discussions hereinabove.

The foregoing measurement may be repeated periodically, allowing enoughtime in between each measurement to insure that the acoustic echosproduced by the transmitted burst die out in the medium of the test bathbefore the next transmission. In this manner, a series of measurementscan be made, with the stored value in the measuring circuit 94 beingrepeatedly updated for comparison purposes or for continuous display,for recording on a chart recorder, or the like.

When it is desired to provide a meter scale calibration signal at theoutput of the amplitude measuring circuit 94, the gating logic 66 shiftsselector switch 62 to the scale calibration terminal 60. In this mode,the drive signal output from the high frequency oscillator 50 issupplied directly to the measuring circuit 94, which then produces onoutput line 100 a sample of the A.C. amplitude of source 50. If thesource voltage E_(in) is substituted for the term I_(out) R_(t) in Eq.(13) it will be seen that the output from the measuring circuit 94 willbe a full scale signal corresponding to no acoustic loss, therebycalibrating the full scale range of the meter, recorder chart or otherdisplay.

Referring to FIG. 5, there is illustrated in block diagram form suitablecircuitry for use in measuring acoustic pressure and intensity and forobtaining a terminated microphone and speaker sensitivity calibration inthe self-reciprocity mode. Although this circuit is slightly differentfrom the circuit of FIG. 4, common elements will be indicated by commonnumbers. As illustrated, then, the high frequency oscillator 50 isconnected through line 54 to the transmit terminal 56, but in thisembodiment, the connection is made through an impedance 120 having aresistance R_(s). The oscillator is also connected to terminal 60 inselector switch 62. The receive terminal 70 in transmit-receive selectorswitch 58 is connected through line 71 and through a sensing impedance122, having a resistance R_(s), to ground line 90. Terminal 70 is alsoconnected by way of line 124 to the measure terminal 76 of switch 62.Finally, switch arm 64 is connected through an impedance 80' to thedrive input of transducer 86 by way of line 84, with impedance 80'having a resistance R_(t) -R_(s). Thus, it will be seen that in thiscircuit the resistance of the terminating impedance 80' has been reducedby the value R_(s), which resistance is instead connected betweenreceive terminal 70 and ground line 90, with an identical resistanceR_(s) being connected between oscillator 50 and terminal 56. Thisinsures that the terminating impedance for transducer 86 issubstantially identical in both the transmit and the receive modes. Itshould be noted that the terminating impedance can be variable, with itsvalue being selected to optimize transducer performance.

As indicated in Eq. (6), (7) and (14)-(17), the measurement of pressure,intensity, and terminated microphone and speaker sensitivity requiresthat the current through the terminating impedance be measured, insteadof the voltage developed across its resistance. In the transmit mode ofthe circuit of FIG. 5, selected by the gating logic 66, the drivingvoltage from oscillator 50 is fed through impedance 120, switch 58, andimpedance 80' to the transducer to generate the acoustic wave 102, asbefore. The return echo 108 then excites the transducer to produce areceiver current in line 84 when the gating logic 66 shifts switch 58 tothe receive mode, and the current on line 84 is fed through impedance80' and impedance 122 to the ground line 90. Impedance 122 is a small,fixed sensing resistor through which the transducer output current flowsto develop a voltage which is applied by way of line 124 and throughswitch 62 as a test signal to the input of the A.C. voltage amplitudemeasuring circuit 94. The sample-and-hold circuit in measuring circuit94 then operates as before to produce a transducer performancecalibration signal on line 100 which is proportional to acousticintensity. This output signal is further applied by way of line 115 tosquare root circuit 116 in order to produce another output signal line118. The signal on line 118 is proportional to acoustic field pressure,microphone and speaker sensitivity, with proportionality constants givenby the square roots of Eqs. (14), (6) and (7), respectively. Theoperation of the gating logic and the sequence and timing of the circuitoperation are the same as discussed above with respect to FIG. 4.

FIG. 6 discloses a further modification of the circuit of FIG. 4, butadapted for the measurement of transducer characteristics and thus forthe calibration thereof when the transducers are connected in atwo-transducer transmit-receive chain of the type illustrated in FIG. 3.In this arrangement, two transducers, 130 and 132 are acousticallycoupled with each other within a test bath 134 containing anacoustically conductive medium. A driving voltage supplied by a highfrequency oscillator or other suitable source 136 is applied by way ofoscillator output line 138 and terminating impedance 140 (having aresistance R_(t)) to the driving input 142 of transducer 130. The outputline 138 of source 136 is also connected by way of line 144 to a scalecalibrating terminal 146 of a selector switch 148. Again, switch 148 maybe a solid state device driven by suitable gating logic, but forconvenience of illustration is shown in its mechanically equivalentform. The reference side of source 136 is connected by way of line 150to a suitable ground point 152. In addition, the source 136 provides online 97 suitable out-of-phase standard signals, to be described.

Upon activation of oscillator 136, a drive signal is applied totransducer 130 which responds to produce an acoustic wave output 154which travels through the acoustically transmissive medium in the testbath 134 and is received by transducer 132. The receive transducerresponds to the acoustic wave to produce an output signal with respectto ground line 164 on its output line 156 which is connected to a firstmeasuring terminal 158 in switch 148. Line 156 is also connected to aterminating impedance in the form of a voltage divider, consisting ofthe series connection of impedance 160 and sensing resistor 162 to aground line 164, which is connected to ground point 152. Impedance 160has a resistance value R_(t) -R_(s) while impedance 162 has a resistancevalue R_(s), the total impedance R_(t) being identical to the valueR_(t) of impedance 140. The junction between impedances 160 and 162 isconnected by way of line 166 to a second measuring terminal 168 inswitch 148.

The switch arm 170 of switch 148 is movable to select any one of theterminals 146, 158 or 168, to connect the selected terminal by way ofline 172 to the input of an A.C. voltage amplitude measuring circuit174. The measuring circuit 174 is voltage-referenced to ground line 164,is connected to voltage source 136 by way of line 97, and produces online 176 a transducer performance calibration signal for thedetermination of transducer sensitivity when switch arm 170 selectsterminal 158, and of acoustic intensity when switch arm 170 connects toterminal 168. Again, these output signals can be further processed in asquaring circuit or a square root circuit to produce output signals forthe measurement of transducer gain, microphone and speaker sensitivity.

The amplitude measuring circuit 174 may be switched periodically toterminal 146 to receive and measure the ideal driving source voltagefrom source 136 to calibrate the display or recording devices aspreviously explained. However, the measuring circuit may also beconnected, by way of measuring terminal 158, to measure the voltagedeveloped across the transducer terminating resistance, which is the sumof impedances 160 and 162, for transducer sensitivity measurements.Finally, the measuring circuit may be connected by way of measuringterminal 168 to measure the voltage developed across the small sensingresistor 162 to obtain a voltage proportional to the current output from132, whereby pressure, intensity and terminated microphone and speakersensitivity measurements may be made for the transmit-receive chain.

A preferred form of the A.C. voltage amplitude measuring circuitillustrated in block diagram form in FIGS. 4, 5, and 6 is shown in FIG.7, the illustrated circuit having particular reference to the FIG. 4circuit for convenience of illustration, but being equally applicable tothe circuits of FIGS. 5 and 6. As illustrated, the drive source consistsof the high frequency oscillator 50 which produces the sine wavetransducer driving output on line 54 which is supplied to the switches58 and 62 in the FIG. 4 circuit for driving the transducer in thetransmit mode of the circuit. The high frequency oscillator alsoprovides on line 97, which includes lines 178 and 179, two square waveoutputs having a 90° phase difference in their zero crossings. Thesesquare waves are applied to corresponding inputs of a pair of identicalsynchronous detectors 180 and 182, which are conventional detectorstages such as the Motorola MC 1496. The transducer test signal to bemeasured is supplied by way of line 92 to the input of a high frequencyreceiver amplifier 184 in the measuring circuit 94. The gain ofamplifier 184 is selectable in steps by means of a range switch 186 toadjust the resolution of the measuring circuit to the level of the inputsignal being measured.

The output of receiver amplifier 184 is applied by way of lines 188 and190 to second inputs on synchronous detectors 180 and 182 respectively,where the received signal is mixed with the two 90° phase shiftedreference signals on lines 178 and 179. The outputs of the detectors 180and 182 which appear on lines 192 and 194, respectively, are lowfrequency signals representing vector components of the received signalamplitude. These low frequency outputs are applied to a VectorComputation Circuit 196, (see, for instance, Analog Devices model AD531data sheet) which retrieves the received signal amplitude from the twovector components of that signal. The received signal appears on outputline 198 which is connected to a sample-and-hold circuit 200 whichpreferrably incorporates a low pass averaging network to obtain anaverage value of the received signal amplitude. The sample-and-holdcircuit 200, under the control of the gating logic 66, samples thereceived signal at a selected time determined by the transmit-receivedelay time, and this sample value is supplied on the measuring circuitoutput line 100 as previously explained. This output signal is thusindicative of various transducer calibration parameters as describedwith respect to FIGS. 4, 5, and 6.

Although the present invention has been described in terms of preferredembodiments, numerous variations can be made without departing from thetrue spirit and scope of the invention. For example, the driving sourcewith its series impedance illustrated in FIG. 6 can be replaced by theNorton-equivalent current source and parallel impedance. Further,instead of a single terminating impedance switched between the transmitand receive modes in FIGS. 4 and 5, two equal termination impedances canbe used, one in the transmit signal path and one in the receive signalpath. Instead of employing a measure of the amplitude of the drivingsource to provide a meter scale calibration signal at the outputs of thecalibration apparatus, this amplitude can instead be continuously orperiodically monitored and employed to internally adjust the gain of thetransducer calibrating apparatus in order to provide a calibrated outputdisplay. While the transducer terminations in these preferredembodiments is often shown as being resistive, complex impedances may beused. In, for instance, FIG. 5 resistance 80' can be replaced by acomplex impedance. Or, in FIG. 4, where the voltage across the resistivepart of the termination is to be measured, resistance 80 may beparalleled by a reactance, and a reactive series branch may be insertedin line 84, without affecting the validity of the calibrationmeasurement. Additional variations will be apparent to those of skill inthe art, and accordingly it is desired that the scope of the inventionbe limited only by the following claims:

What is claimed is:
 1. A self-reciprocity calibration method forterminated ultrasonic transducers, comprising:supplying a driving signalfrom a driving source to a transducer in a transmit mode to cause saidtransducer to emit a burst of ultrasonic waves; switching saidtransducer to a receive mode to receive an echo of its emitted wave,said transducer responding to said echo wave to produce a transduceroutput signal; measuring the ideal source value of said driving signalsource; measuring said transducer output signal, said driving signal andsaid output signal being fed through substantially identical impedancevalues; and deriving from the measures of said driving signal and saidoutput signal a measure of transducer performance in absolute termsunder conditions of actual termination.
 2. The method of claim 1,wherein said driving signal and said transducer output signal are fedthrough the same terminating impedance, said impedance being connectedalternately in the transmit and receive circuits of said transducer. 3.The method of claim 1, wherein said driving signal and said transduceroutput signal are fed through different terminating impedances havingsubstantially the same impedance values.
 4. The method of claim 1,wherein the step of measuring said transducer driving signal sourcecomprises measuring the open circuit voltage thereof, and the step ofmeasuring said transducer output signal comprises measuring the voltagethereof produced across the resistive part of said transducer receivingtermination impedance, in order to obtain a performance calibrationsignal value which is a measure of transducer sensitivity.
 5. The methodof claim 1, wherein the step of measuring said transducer driving signalsource comprises measuring the square of the open circuit voltagethereof, and the step of measuring said transducer output signalcomprises measuring the square of the voltage thereof produced acrossthe resistive part of said transducer receiving termination impedance,in order to obtain a performance calibration signal value which is ameasure of transducer gain.
 6. The method of claim 1, wherein the stepof measuring said transducer driving signal source comprises measuringthe open circuit voltage thereof, and the step of measuring saidtransducer output signal comprises measuring the current thereof, inorder to obtain a performance calibration signal value which is ameasure of the acoustic field intensity.
 7. The method of claim 1,wherein the step of measuring said transducer driving signal sourcecomprises measuring the square root of the open circuit voltage thereof,and the step of measuring said transducer output signal comprisesmeasuring the square root of the current thereof, in order to obtain aperformance calibration signal value which is a measure of pressure,terminated microphone sensitivity and terminated speaker sensitivity. 8.The method of claim 6, wherein the step of measuring the transduceroutput current includes feeding said output signal though a firsttermination impedance and through a second sensing resistance of knownvalue connected in series therewith, and measuring the voltage dropacross said second resistance, said voltage drop being proportional tosaid output current.
 9. The method of claim 8, wherein said drivingsignal is fed through said first termination resistance and through athird matching resistance connected in series therewith during saidtransmit mode, said third resistance being substantially equal in valueto said second resistance.
 10. The method of claim 1, wherein the stepof measuring said transducer output signal includes deriving a testvoltage value proportional to the current or to the voltage of saidtransducer output signal, comparing said test signal with first andsecond standard signals derived from said driving source to producefirst and second vector component signals for said test voltage, andderiving from said first and second vector component signals a resultantsignal proportional to the amplitude of said transducer output signal.11. The method of claim 10, further including sampling and storing anaveraged value of said resultant signal, said stored value comprisingsaid transducer output signal.
 12. A calibration method for terminatedultrasonic transducers comprising:continuously supplying from a drivesignal source a driving signal to a first transducer to be tested in atransmit mode to cause said first transducer to emit ultrasonic waves;mounting a second transducer to be tested in the path of said ultrasonicwaves, said second transducer operating in a receive mode to respond tosaid ultrasonic waves to produce a transducer output signal; measuringthe ideal source value of said first transducer driving signal source;measuring said second transducer output signal, said driving signal andsaid transducer output signal being fed through substantially identicalterminating impedance values connected to said transducers, whereby theperformance characteristics of said transducers are measured in absoluteterms as a function of the applied terminating impedances; and derivingfrom said measures of transducer driving signal and transducer outputsignal a transducer performance calibration signal under conditions ofactual termination.
 13. The method of claim 12, wherein said drivingsignal for said first transducer and said second transducer outputsignal are fed through different terminating impedances havingsubstantially identical impedance values.
 14. The method of claim 12,wherein the step of measuring said first transducer driving signalsource comprises measuring the open circuit voltage thereof, and thestep of measuring said second transducer output signal comprisesmeasuring the voltage thereof produced across the resistive part of saidsecond transducer terminating impedance, in order to obtain aperformance calibration signal value from which transducer sensitivitycan be derived.
 15. The method of claim 12, wherein the step ofmeasuring said first transducer driving signal source comprisesmeasuring the square of the open circuit voltage thereof, and the stepof measuring said second transducer output signal comprises measuringthe square of the voltage thereof produced across the resistive part ofsaid second transducer terminating impedance, in order to obtain aperformance calibration signal value which is a measure of transducergain of the two-transducer network.
 16. The method of claim 12, whereinthe step of measuring said first transducer driving signal sourcecomprises measuring the open circuit voltage thereof, and the step ofmeasuring said second transducer output signal comprises measuring thecurrent thereof, in order to obtain a performance calibration signalvalue from which the acoustic field intensity can be derived.
 17. Themethod of claim 12, wherein the step of measuring said first transducerdriving source comprises measuring the square root of the open circuitvoltage thereof, and the step of measuring said second transducer outputsignal comprises measuring the square root of the current thereof, inorder to obtain a performance calibration signal value from whichacoustic pressure, terminated microphone sensitivity and terminatedspeaker sensitivity can be derived.
 18. The method of claim 16, whereinthe step of measuring the output current of said second transducerincludes feeding said output signal through a first terminationimpedance and through a second sensing resistance of known valueconnected in series therewith, and measuring the voltage drop acrosssaid second resistance, said voltage drop being proportional to saidoutput current.
 19. A self-reciprocity calibration circuit arrangementfor terminated ultrasonic transducers, comprising:an ultrasonictransducer capable of operating in transmit and receive modes; a highfrequency voltage source producing a drive signal; first circuit meansfor connecting said drive signal to said transducer to cause saidtransducer to operate in its transmit mode to emit acoustic waves;reflector means for reflecting at least a portion of said acoustic wavesback to said transducer to cause said transducer to generate an outputsignal; second circuit means connectable to said transducer to causesaid transducer to operate in its receive mode and for receiving saidtransducer output signal; terminating impedance means included in saidfirst circuit means during the transmit mode of said transducer, and insaid second circuit means during the receive mode of said transducer;measuring circuit means connected to said first circuit means andresponsive to the ideal source value of said voltage source andconnected to said second circuit means and responsive to said transduceroutput signal; and output circuit means responsive to said measuringcircuit means to produce a transducer performance calibration signalwhich is a measure of transducer performance in absolute terms underconditions of actual termination.
 20. The calibration circuitarrangement of claim 19, wherein said terminating impedance meansincludes a first terminating impedance connected in said first circuitmeans and a second terminating impedance having a value substantiallyidentical to the value of said first terminating impedance connected insaid second circuit means.
 21. The calibration circuit of claim 20,wherein said first and second circuit means further include switch meansto shift said transducer between its transmit and receive modes, andwherein said calibration circuit arrangement contains means forcontrolling said switch means.
 22. The calibration circuit of claim 19,wherein said measuring means comprises measure select circuit means toselectively connect said measuring means to either said first or saidsecond circuit means, whereby said output circuit means is selectivelyresponsive to either a scale calibration value or a transducer outputsignal value.
 23. The calibration circuit arrangement of claim 19,wherein at least part of said first and second terminating impedancesare common to said first and second circuit means.
 24. The calibrationcircuit arrangement of claim 19, wherein said measuring circuit means isconnected to said second circuit means to measure the voltage producedby said transducer output signal across a resistive part of said secondterminating impedance.
 25. The calibration circuit arrangement of claim19, wherein said measuring circuit means is connected to said secondcircuit means to measure said transducer output signal current.
 26. Thecalibration circuit of claim 25, wherein said second terminatingimpedance includes sensing means to measure said transducer outputsignal current.
 27. The calibration circuit arrangement of claim 19,wherein said measuring circuit means is further connected to a standardvoltage produced by said high frequency voltage source, whereby saidtransducer output signal is combined with said standard voltage toproduce a measure of said transducer output signal.
 28. The calibrationcircuit of claim 27, wherein said measuring circuit includes quadraturedetector means for combining said transducer output signal with saidstandard signal, means responsive to said quadrature detector means forproducing a resultant signal proportional to the amplitude of saidtransducer output signal, and sample and hold means for storing theaverage value of said resultant signal.
 29. A calibration circuitarrangement for terminated ultrasonic transducers, comprising:first andsecond ultrasonic transducers mounted in spaced apart relationship in anacoustic test fixture and defining an acoustic path; a high frequencydrive voltage source; first circuit means having a transmit mode andincluding a first termination impedance, said first circuit meansconnecting said drive source through said first termination impedance tosaid first transducer to cause said first transducer to emit acousticwaves along said acoustic path to impact said second transducer and tocause said second transducer to produce an output signal; second circuitmeans having a receive mode and including a second termination impedanceconnected to said second transducer, said second termination impedancehaving a value substantially equal to the value of said firsttermination impedance, said second circuit means receiving saidtransducer output signal; measuring circuit means connectable to saidfirst circuit means and responsive to the open circuit amplitude valueof said drive source and connectable to said second circuit means andresponsive to said transducer output signal; and output circuit meansresponsive to said measuring circuit means to produce a transducerperformance calibration signal.
 30. The calibration circuit of claim 29,said measuring means including measure select circuit means toselectively connect said measuring means to either said first or saidsecond circuit means, whereby said output circuit means is selectivelyresponsive to either a scale calibration value or a transducer outputsignal value.
 31. The calibration circuit arrangement of claim 29,wherein the real part of said second termination impedance consists of asensing resistance of known value in series with the remainder of itsresistance, and wherein the second circuit means further includes switchmeans for selectively connection said measuring circuit means acrosseither said sensing resistance alone or across the series connection ofsaid termination resistances.
 32. The calibration circuit arrangement ofclaim 31, wherein said measuring circuit means is further connected to astandard voltage produced by said high frequency voltage source, wherebysaid transducer output signal is combined with said standard voltage toproduce a measure of said transducer output signal.
 33. The calibrationcircuit arrangement of claim 32, wherein said measuring circuit includesquadrature detector means for combining said transducer output signalwith said standard signal and means responsive to said quadraturedetector means for producing a resultant signal proportional to theamplitude of said transducer output signal.